Relationship of arsenic accumulation with irrigation practices and crop type in agriculture soils of Bengal Delta, India

The present study investigates the current state and distribution of As in the upper soil horizons (i.e. rhizospheric zone, 0-15 cm) of two different agricultural fields: (1) paddy-cultivating agricultural field that was irrigated with groundwater containing 137 mu g/L of As and (2) wheat-cultivating agricultural field that was irrigated with groundwater having 67.3 mu g/L of arsenic. Results clearly indicate different levels of As accumulation in the upper soil horizons of both profiles. In paddy field, although significantly higher quantity of As-contaminated groundwater was used for irrigation, still lesser than expected As concentration in soils was found [average As concentration 16.0 mg/kg (measured) vs. 29.0 mg/kg (calculated)]. The imbalance between higher influx of As and its relatively lower accumulation in soils indicates the existence of a mechanism (organic carbon mechanism, elaborated in the main text) that is responsible for continuous removal of As, and ultimately prevents the expected shoot-up of As in the paddy soils. On the other hand, although lesser quantity of less contaminated groundwater is used in wheat field, still wheat field soils show relatively higher As accumulation [average As concentration 22.5 mg/kg (measured) vs. 12.2 mg/kg (calculated)]. Such accumulation of As happens when there is continuous influx of As through irrigation water and/or other sources, and an effective (natural) mechanism to remove As from the wheat soil is absent. Adoption of distinct harvesting methods is responsible for existence of different mechanisms in paddy and wheat fields, which ultimately cause the differential accumulation of As in paddy and wheat soils.